1. Field of the Invention
The present invention relates to linear or rotary optical encoders used to measure the position of a linear or rotary moving element such as an actuator.
2. Description of the Prior Art
It is certainly true that optical encoders are well known in the prior art. A typical optical encoder would be utilized to measure the digital absolute position of a linear or rotary moving element. Generally, these prior art optical encoders would include a code plate having several tracks of alternating dark and transparent bands and an optical emitter and detector on either side of the code plate assigned to reading each band. Alternatively, the emitter and detector may be on the same side of the code plate with the alternating bands having high and low reflectivity. In either situation, an optical emitter/detector combination would be assigned to each of the bands, thereby increasing the complexity and the cost of the system. A typical optical reading encoder capable of resolving the absolute position of the code plate to one part in a thousand would require approximately 10 pairs of optical emitters and detectors to read the 10 tracks on the code plate.
Examples of prior art optical detectors are U.S. Pat. No. 4,602,242 issued to Kimura; U.S. Pat. No. 4,720,699 issued to Smith; U.S. Pat. No. 5,336,884 issued to Khoshnevisan et al.; U.S. Pat. No. 5,451,776 issued to Kolloff et al.; and U.S. Pat. No. 5,748,111 issued to Bates.
The patent to Smith describes an optical encoder employing a line array of detectors. A single strip of encoding bars is used in conjunction with a plurality of detectors 6 to determine the absolute position of the strip of encoding bars. As shown in FIG. 4, a multitude of detector element 6 must be utilized.
The patent to Kolloff et al. describes an optical encoder for sensing the linear position of an optical medium moving in a linear direction. As illustrated in FIGS. 1 and 2, the encoder includes a rectangular plate 102 provided with a linear absolute track 104 as well as additional linear tracks 110, 112, 114 and 116. Each track is associated with 1 of 5 optical fibers 188, 190, 192, 194 and 196 extending from a distant point with respect to the encoder assembly to connect with other elements. Each of the tracks contains a series of optically detectable marks spaced evenly around the rectangular plate under each of the tracks. Each of the marks is detected by interferometric reflection of coherent light.
The patent to Khoshnevisan et al. is generally related to an optic encoder for sensing an angular position. A circular disc is provided with an angular absolute track 104 as well as a number of additional angular tracks 110, 112, 114 and 116 as illustrated in FIG. 1. Each of the tracks is provided with a plurality of angularly-spaced “dark” or “bright” bits. A plurality of optical fibers 188–196 is used, one optical fiber associated with each of the tracks.
The patent to Kimura describes either a linear or rotary encoder containing a code carrying medium having a fine reading track 21 and a rough reading track 22 which is parallel with respect to one another. The fine reading track 21 is divided into a plurality of blocks 21a and the rough reading track 22 is divided into a plurality of blocks 23.
The patent to Bates describes an apparatus for monitoring the speed and actual position of a rotating member. This apparatus includes a grating portion 114 including a plurality of wedge-shaped pattern markings 22 that are equally spaced around a periphery surface 20. A sensor 16 in conjunction with a processor 18 would determine both the speed and actual position of the rotating member.
None of the above-noted prior art patents describes an optical encoder utilizing a code plate having a plurality of wedge-shaped items defining a plurality of digital words associated with a linear or rotary moving element. Additionally, none of these references would employ a relatively small number of detectors to quickly and efficiently detect the absolute position of the code plate which, in turn, is associated with the actual position of the linear or rotary moving element.